In current ROADM flexibility is only provided in handling wavelength channels coming from the network (by-pass wavelengths also known as express traffic) that can be remotely configured at each node to be routed from any direction to any direction. But at the end points the add and drop wavelengths are rigidly assigned to a fixed direction and can only be reconfigured manually.
Next generation ROADM will be more flexible with respect to the currently deployed optical nodes. New ROADM will have colourless, directionless and contentionless features so as to extend the flexibility and automation to the end points where normally the transponders are connected to the node. In new nodes it will be possible without any manual intervention to change the configuration of add/drop wavelength channels to/from any direction (directionless operation), independently from the transponder wavelength (colourless operation) and by allowing multiple signals with the same wavelength to be handled by the same add and drop structure (contentionless operation).
The dynamic colourless, directionless and contentionless add/drop access will give the operator the possibility to optimize the resources utilization, eliminate manual intervention, support re-routing functions in case of faults in a cost effective way. To add such flexibility to the existing ROADMs that uses the free space optics based 1×N wavelength selective switching (WSS) for optical line switching, new architectures have been presented and illustrated in this document in FIG. 1. The architecture illustrated in FIG. 1 allows for the colourless, directionless and contentionless operation thanks to the use of an additional element named Add and Drop Switch and Aggregators block (ADSA). Three main possible realizations have been discussed in the literature for ADSA:                1) Solution based on arrays of 1×N WSS in conjunction with arrays of 8×1 space switching as disclosed in P. Colbourne, B. Collings: ‘ROADM Switching Technologies’ Proceeding of OFC 2011, paper OTuD1.        2) Solution presented in FIG. 5 of S. Gringeri et al: ‘Flexible Architectures for Optical Transport Nodes and Networks’ IEEE Communication Magazines, July 2010, which is based on distribute-switch-select architecture in which splitters/combiners are used in conjunction with 1×N optical switches and tunable filters.        3) Solution presented in R. Jensen, A. Lord and N. Parsons: ‘Colourless, Directionless, Contentionless ROADM Architecture Using Low-Loss Optical Matrix Switches’, Proceeding of ECOC 2010 paper Mo.2.D.2 in which ADSA blocks include high port count OXC based on free space optics and MEMS technology.        
All these proposed architectures are not satisfactory: the architecture 1 has low losses but has the disadvantage of high cost, room occupancy and complexity due to the high number of WSS (wavelength selective switches) in the array that increases scaling the transponder count. The architecture 2 has the disadvantage of high losses that increase scaling the transponder count, due to signal distribution and switching. This leads to increase of ROADM cost, footprint and power consumption (needs of optical amplification). The architecture 3 has the disadvantage of high cost and large footprint due to the use of 3D MEMS (microelectromechanical systems) technologies with the related complex control circuitry.